The current heightened sense of security and declining cost of camera equipment have resulted in increased use of closed circuit television (CCTV) surveillance systems. Such systems have the potential to reduce crime, prevent accidents, and generally increase security in a wide variety of environments.
A simple closed-circuit television system uses a single camera connected to a display device. More complex systems can have multiple cameras and/or multiple displays. One known type of system is the security display in a retail store, which switches periodically between different cameras to provide different views of the store. Higher security installations, such as prisons and military installations, use a bank of video displays each displaying the output of an associated camera. A guard or human attendant constantly watches the various screens looking for suspicious activity.
More recently, inexpensive digital cameras have become popular for security and other applications. In addition, it is now possible to use a web cam to monitor a remote location. Web cams typically have relatively slow frame rates, but are sufficient for some security applications. Inexpensive cameras that transmit signals wirelessly (e.g., ×10 cameras) and over power cables to remotely located computer or other displays are also used to provide video surveillance.
As the number of cameras increases, the amount of raw information that needs to be processed and analyzed also increases. Computer technology can be used to alleviate this raw data processing task, resulting in a new breed of information technology device—the computer-aided surveillance (CAS) system. Computer-aided surveillance technology has been developed for various applications. For example, the military has used computer-aided image processing to provide automated targeting and other assistance to fighter pilots and other personnel. In addition, computer-aided surveillance has been applied to monitor activity in swimming pools.
A CAS system automatically monitors objects (e.g., people, inventory, etc.) as they appear in series of surveillance video frames. One particularly useful monitoring task is tracking the movements of objects in a monitored area. Methods for tracking objects, such as people, moving through an image are known in the art. To achieve more accurate tracking information, the CAS system can utilize knowledge about the basic elements of the images depicted in the series of surveillance video frames.
On a macroscopic level, a video surveillance frame depicts an image of a scene in which people and things move and interact. On a microscopic level, a video frame is composed of a plurality of pixels, often arranged in a grid-like fashion. The number of pixels in an image depends on several factors including the resolution of the camera generating the image, the display on which the image is presented, the capacity of the storage device on which the images are stored, etc. Analysis of a video frame can be conducted either at the pixel level or at the (pixel) group level depending on the processing capability and the desired level of precision. A pixel or group of pixels being analyzed is referred to herein as an “image region.”
Image regions can be categorized as depicting part of the background of the scene or as depicting a foreground object. In general, the background remains relatively static in each frame. However, objects are depicted in different image regions in different frames. Several methods for separating objects in a video frame from the background of the frame, referred to as object extraction, are known in the art. A common approach is to use a technique called “background subtraction.” Of course, other techniques can be used. The locations of the objects are typically recorded in a list that is associated with the video frame.
To track an object from frame to frame, a tracking method determines a correspondence between extracted objects in the current frame and extracted objects in the previous frame. This correspondence can be determined, for example, by using a predictive tracking method. The CAS system predicts the location of an object in the current frame based on the known locations of the object in previous frames. Subsequently, the predicted object location is compared to the actual object location to establish correspondence. Such a prediction is typically based on an algorithm that predicts likely object movement. For example, it can be assumed that objects move with constant velocity. More sophisticated techniques can, for example, verify that the colors of the objects match before determining a correspondence.
While conceptually simple, a robust tracking system faces many difficulties. Changes in scene lighting can affect the quality of object extraction, causing foreground elements to be misshapen or omitted completely. Object occlusions can cause objects to disappear or merge together, leading to difficulties in correspondence between frames. The tracked objects can change shape or color over time, preventing correspondence even though the objects were properly extracted.
In addition, even under ideal conditions, single-view tracking systems invariably lose track of monitored objects that leave the field-of-view of the camera. When multiple cameras are available, as in many close-captioned television systems, it is theoretically possible to reacquire the target when it appears in a different camera. This ability to perform automatic “sensor hand-off” is of significant practical interest. Current laboratory solutions require geometrically calibrated cameras with overlapping fields-of-view, conditions that are not readily achieved in typical CCTV installation.